In OFDM-IM, there are two types of error events, an index demodulation error event and a symbol error event. The former is the error event when the SAP is incorrectly detected and the latter is the error event when the modulated symbols in the active subcarriers are incorrectly detected though the SAP is correctly detected.
As described in [1]}, the symbol error event has the diversity order of one and the index demodulation error event has the diversity order of two. By virtue of the frequency diversity gain, the index demodulation error event occurs less frequently than the symbol error event in high SNR region. That is, the symbol error event dominates the overall error performance of the OFDM-IM system in high SNR region.
Therefore, in [2]} there is an attempt to reduce PAPR by inserting the dither signals in the idle subcarriers. Clearly, this dither signal does not affect the symbol error event and only degrades the index demodulation error performance.
Also, the authors in [2]} introduces an equivalent amplitude constraint for the dither signals in the idle subcarriers.
